This section introduces aspects that may help facilitate a better understanding of the disclosure. Accordingly, these statements are to be read in this light and are not to be understood as admissions about what is or is not prior art.
Hydrothermal pretreatments, such as liquid hot water pretreatment, steam explosion, and dilute acid pretreatment, enhance cellulolytic conversion of lignocellulose by solubilizing the hemicellulose fraction of the biomass. Due to the distinctively different pretreatment optima for hemicellulose and cellulose, a multi-stage pretreatment offers advantages over a single-stage pretreatment in terms of achieving a maximal recovery of both pentose and hexose sugars. In a multi-stage pretreatment, the first pretreatment can be optimized to remove the majority of hemicellulose while minimizing degradation of hemicellulose and re-precipitation of humins derived from the degradation. The subsequent pretreatments are more targeted to generate cellulose with enhanced susceptibility to cellulases.
The extent of hemicellulose removal alone is not a reliable parameter to predict enzymatic hydrolyzability of pretreated mixed hardwood. A great degree of variability in cellulose conversion yields existed among the pretreated hardwood materials even if majority of initial xylan was removed from the lignocellulose during liquid hot water pretreatment.
Given the integrative nature of the cell wall structure [67], pretreatment that changes one property will also affect others. Removal of hemicellulose and some lignin during liquid hot water pretreatment of lignocellulose, therefore, would also alter other substrate characteristics that affect cellulose hydrolyzability. The ease of enzymatic hydrolysis of cellulosic feedstocks can be affected by several substrate characteristics, such as cellulose's accessibility to cellulases which is a function of the degree of polymerization (DP), crystallinity, lignin and hemicellulose content and structure, and particle size.
Studies have investigated the effect of swelling, DP, fiber size, enzyme accessible area, crystallinity, composition, and presence of lignin on the enzymatic hydrolysis of various lignocellulosic materials. However, most studies involved measuring substrate characteristics of lignocellulose feedstocks that were either pretreated by different pretreatment technologies or pretreated within a limited range of pretreatment conditions.
There is therefore an unmet need for strategies to mitigate the inhibitory effects of lignin and soluble phenolics to ensure robust and economic enzymatic conversion of cellulose.